You will find that people who've never been on a cruise know that they are horrible, boring and disease-ridden. Full of people they won't like that they can't get away from.
OTOH, it's rare for people to go on only one cruise - they tend to like cruising so much that they keep booking.
It's hard to describe sea days because they are what you make of them. You can get up late, wander into breakfast, then to coffee, lunch, afternoon tea, dinner and pre-bed snacks. You can spend hours in an air-conditioned bar listening to your preferred type of music. You can soak up the sun on deck, with drinks being brought to you - I've never not managed to find a sunbed, either in the midst of things or in a quiet corner, according to how I feel, or you can sit at a table. You can swim, soak in a jacuzzi, join in activities: sports, choirs, quizzes, creative, etc. You can listen to talks - various subjects, some more interesting than others. You can go to the cinema, or watch a choice of films on the TV in your room. You can sit on deck or your balcony and watch the sea go by, or sit in a communal area and make it obvious whether you'd like to chat or be left alone - people generally seem to read others' attitudes well. In the evening there's the main show, which varies from day to day, or dancing, disco, karaoke or various types of music in the bars.
I guess if you tend to be bored generally you could be bored on a cruise ship. But doing any activity requires minimal effort - you don't have to drive or catch a bus to a venue.